Global Positioning Satellite (GPS) navigation as exemplified by NAVSTAR/GPS, is an accurate, three-dimensional navigation system which has become one of the most important technologies of the era, impacting a myriad of users from aircraft and ships, to farmers and hikers. The GPS comprises a constellation of twenty four satellites and three spares which orbit the earth twice a day. The orbits of the GPS satellites are maintained in a virtually circular manner at approximately 10,898 nautical miles above the earth, the GPS satellites orbiting the earth in six overlapping orbital planes based on the equatorial plane of the earth. These orbits are chosen so that the GPS system can provide information to users regardless of the time that the user requests information and regardless of the user's position on the earth's surface. This information contains a navigation message, which includes satellite ephemerides and satellite clock drift information.
Accordingly, the Global Positioning System (GPS) enables user position information to be easily and automatically transmitted in voice and data messages to support situation awareness (SA) in military tactical environments using for example, a United States (US) Army, Single Channel Ground-Airborne Radio System (SINCGARS) Combat Net Radio (CNR). The SINCGARS is a frequency hopping radio network system that hops over the 30-88 MHZ band, with 25 kHz frequency spacing and thus, enables GPS-based range measurements to be made even while transmitting. For more information, see Robert C. Dixon, Spread Spectrum Systems (second edition, 1984). The existing US Army CNR provides GPS user position information, attached to voice and data messages and employs user position transmitting techniques which initiate stand-alone SA GPS position messages based on user movement or elapsed time so that position updates are not totally dependent on voice or data transmissions.
The SA GPS position messages are initiated by the existing US Army CNR in two different user-selectable modes of operation. The first mode of operation is based on radio movement (referred to as the "movement" mode), where a SA message is sent when the CNR has moved either 100 meters (manpack configuration) or 300 meters (vehicular configuration). The second mode of operation is based on elapsed time (referred to as the "periodic" mode), where a SA message is sent every two minutes. The radio operator can select either mode of operation (but not both) from the front panel of the CNR. In addition, the radio operator can turn the GPS function off or set the CNR to send position information only attached to voice and data messages but not as a stand alone SA message (AUTO position). The GPS mode front panel indications and corresponding functions for a prior art CNR are shown in FIG. 1.
There are several problems associated with the US Army implementation of the SA position message reporting in the "movement" and "periodic" modes of operation. In the "movement" mode of operation, the first problem associated therewith relates to the decreasing number of automatic position updates which occur as the CNR moves slower. When the CNR is in the "movement" mode, there are no stand alone SA position messages generated when the CNR is stationary. That is, if the person or vehicle carrying the CNR is not moving for any period of time, there will be no SA position messages generated by the CNR. If voice and data messages are not being sent on a regular basis, and the CNR is not moving, the user's position will not be sent.
The second problem associated with the "movement" mode of operation is that the faster the vehicle is moving, the more often the automatic position updates are generated. Since the movement threshold is set to 300 meters, vehicles moving at a faster rate of speed will generate SA position messages at an excessively quick rate. For example, vehicles traveling over 30 mph will generate SA position messages faster than once every 15 seconds. This can severely impact SINCGARS (radio net or radio network) loading; especially when multiple vehicles are moving in formation at a fast rate.
In the "periodic" mode of operation the problem associated therewith is that the accuracy of the position reporting becomes significantly less as the CNR moves faster. When the CNR is in the "periodic" mode, SA position messages are sent every two minutes. When a vehicle is moving rapidly, the distance between position updates can become large. For example, a vehicle traveling 25 miles per hour will transmit periodic position updates over 1.3 km apart. At 40 miles per hour, the distance between SA position messages will be greater than 2 km.
Accordingly, an object of the present invention is to provide a method for reducing the number of operating options available to a user of mobile radio device in order to simplify the operation of the device and improve the overall performance of the device in most of its applications.
Another object of the present invention is to provide a mobile radio device which employs the above method.